1. Field of the Invention
The invention concerns a method for the correction of image distortions that occur in exposures of diffusion-weighted magnetic resonance images (MR images) of an examination subject, as well as a magnetic resonance system (MR system) for this. The invention is in particular applied in the correction of diffusion-weighted images that were acquired using the echoplanar technique (EPI).
2. Description of the Prior Art
In echoplanar imaging, after a single RF excitation pulse with a series of echoes in the readout gradients signals are generated that can be associated with different lines in k-space by a modulation of the phase coding gradients. Distortions due to eddy currents represent a great challenge in diffusion-weighted EPI imaging since high gradient amplitudes for diffusion imaging (known as the diffusion gradients) are used in combination with a high sensitivity in the phase coding direction, which leads to the distortions. In the phase coding direction, the resolution in such EPI images is typically approximately 10 Hz per pixel.
In diffusion imaging, generally multiple MR images with different diffusion directions and diffusion weightings are acquired and combined with one another in order to calculate parameter maps for diffusion coefficients, for example, such as ADC (Apparent Diffusion Coefficient) or FA (Fractional Anisotropy). The diffusion weighting is described by a b-value that depends on the strength of the applied diffusion gradient and is measured in seconds per square millimeter. These diffusion images with different diffusion directions and weightings can then be used for diagnostic purposes. The eddy current fields that are generated by the diffusion gradients, however, lead to image distortions whose appearance depends both on the amplitude of the gradients (i.e. the diffusion weighting) and on their direction. The distortions can be described in a good approximation as a simple affine transformation with the scaling M, the shear S and the displacement or translation T. If the acquired individual images are combined with one another without correction, the different distortions for each image lead to incorrect associations of pixel information, and therefore to errors or at least to a reduced precision of the calculated parameters.
In the prior art, several image-based methods are known for the correction of eddy current-based distortions in diffusion imaging. For example, it is described in Haselgrove et al. in MRM 36:960-964, 1996 that an MR image with b=0 (i.e. an undistorted image) is acquired which serves as a reference image. Furthermore, an additional adjustment measurement with lower diffusion weighting is acquired for the direction to be corrected, wherein a low diffusion weighting means, for example, a b-value of 150 s/mm2. The distortion parameters M, S and T determined with these measurements are utilized using an extrapolation relationship for the correction of the actual diffusion-weighted MR images in which the b-value is, for example, 1000 s/mm2.
This method has the disadvantage that an adjustment measurement is necessary for every diffusion direction. For a precise conclusion about the diffusion, however, information is also required in a great many different directions, for example between 5 and 200 different directions. Since an adjustment measurement is necessary for every diffusion direction, this would lead to intolerably long acquisition times. In diffusion-weighted images with b=150 s/mm2, the distortions are not yet very strongly pronounced, such that the precise determination of the parameters such as scaling, shearing and translation is difficult. Via the extrapolation of these values to larger b-values, errors in the determination of the b150 MR image are intensified. Movements between the acquisition of the reference image and the adjustment measurement can likewise lead to incorrect determination of the correction parameters. Furthermore, the contrast between the two images is very similar but not identical, which leads to an inadequate robustness of the method as soon as tissue with rapidly diffusing water molecules is present in the image.
Furthermore, in Bodammer et al. in MRM 51:188-193, 2004 it is described that two images with identical diffusion direction and weighting but inverted polarity are acquired. While the diffusion contrast remains unchanged given inverted polarity, the inversion affects the distortion by a compression being produced from a stretching, a negative shear is produced from a positive shear and a negative translation is produced from a positive translation. In this method, two images must be acquired for each diffusion direction and for every diffusion weighting. Moreover, the signal-to-noise ratio in images with high b-values can be extremely high, which makes the precise determination of the correction parameters difficult. Furthermore, contrast differences due to directed movement (for example flow or pulsations) can lead to an inadequate robustness of the method. Furthermore, movements between the acquisition of the two measurements can lead to incorrect determinations of the correction parameters.